Obstructive sleep apnea (OSA), a condition that affects a quarter of American adults, is strongly and independently associated with an increased risk for cardiovascular diseases and increased all-cause mortality. Effective treatment of OSA reduces the risk of cardiovascular diseases in patients with OSA. However, OSA remains frequently unrecognized and a majority of OSA patients do not adhere to standard continuous positive airway pressure therapy. Low rate of OSA recognition and poor adherence with standard therapy underscores the urgent need for novel diagnostic and therapeutic approaches to cardiovascular manifestations of OSA. Repetitive episodes of hypoxia/reoxygenation during transient cessation of breathing in OSA lead to activation of both arterial and venous peripheral endothelium, a key step in the development and progression of cardiovascular diseases. We have developed a minimally invasive technique of endothelial harvesting from a superficial forearm vein that allows safe collection and direct examination of endothelial cells without the artifact of culture conditions in OSA patients. Using freshly harvested endothelial cells, we have demonstrated directly peripheral endothelial activation in OSA patients as evidenced by increased expression of nuclear factor kappa B and nitrotyrosine formation and reduced expression of endothelial nitric oxide synthase. Repetitive hypoxia/reoxygenation, a phenomenon unique to sleep apnea, may activate the peripheral endothelium through specific pathways. The systemic nature of endothelial activation in OSA suggests the presence of circulating endogenous ligands that target and engage endothelial cells. We have identified peptide FHENWPS (Phe-His-Glu-Asn-Trp-Pro-Ser) as a specific ligand that targets and activates endothelial cells in OSA patients prior to onset of clinically evident cardiovascular diseases. This led us to hypothesize that ligand FHENWPS is associated with peripheral endothelial activation in OSA and thereby may accelerate the development and progression of cardiovascular diseases. To address this hypothesis, we are proposing: (1) To determine the presence of ligand FHENWPS in OSA patients without overt cardiovascular diseases before and after CPAP therapy (Aim 1), (2) To determine the effects of ligand FHENWPS on peripheral endothelial activation and systemic inflammation in OSA (Aim 2), and (3) To determine whether ligand FHENWPS augments peripheral endothelial activation and systemic inflammation in patients with OSA and coexistent CAD (Aim 3). Using a novel approach to characterize human vascular endothelium, the proposed studies may advance our understanding of endothelial activation in OSA and may allow (1) early identification of OSA patients who are at risk for vascular diseases and (2) provide basis for functional studies that may lead to the development of novel therapeutic strategies for preventing and/or reversing vascular risk in OSA.